Herd Density Research Articles

An evaluation of two potential risk factors, MHC diversity and host density, for infection by an invasive nematode Ashworthius sidemi in endangered European bison ( Bison bonasus)

Parasites are thought to increase the risk of host extinction, but their dynamics in endangered species have not been well investigated. The free-living European bison population in Białowieża Forest has recently been massively invaded by a blood-sucking nematode, Ashworthius sidemi. This nematode originated in Asia and was probably transmitted to Europe with the introduction of the sika deer. Here, we investigate the impacts of genetic factors (the Major Histocompatibility Complex class II genes responsible for extracellular parasite recognition) and management practices (supplementary feeding affecting winter population density) on the intensity of A. sidemi infections in bison. All but two out of 110 animals investigated between 2005 and 2009 were infected with A. sidemi, and the intensity of infection increased significantly with time. Due to a severe population bottleneck experienced by the bison, only four class II DRB alleles are retained in the Białowieża population. We found that despite high sequence divergence, neither any of the alleles nor DRB heterozygosity was significantly associated with infection intensity. We did find, however, that winter density of bison herds was positively associated with infection intensity. Winter bison population densities were in turn predicted by the intensity of supplementary feeding.

Spatial patterns of Bovine Corona Virus and Bovine Respiratory Syncytial Virus in the Swedish beef cattle population

BackgroundBoth bovine coronavirus (BCV) and bovine respiratory syncytial virus (BRSV) infections are currently wide-spread in the Swedish dairy cattle population. Surveys of antibody levels in bulk tank milk have shown very high nationwide prevalences of both BCV and BRSV, with large variations between regions. In the Swedish beef cattle population however, no investigations have yet been performed regarding the prevalence and geographical distribution of BCV and BRSV. A cross-sectional serological survey for BCV and BRSV was carried out in Swedish beef cattle to explore any geographical patterns of these infections.MethodsBlood samples were collected from 2,763 animals located in 2,137 herds and analyzed for presence of antibodies to BCV and BRSV. Moran's I was calculated to assess spatial autocorrelation, and identification of geographical cluster was performed using spatial scan statistics.ResultsAnimals detected positive to BCV or BRSV were predominately located in the central-western and some southern parts of Sweden. Moran's I indicated global spatial autocorrelation. BCV and BRSV appeared to be spatially related: two areas in southern Sweden (Skaraborg and Skåne) had a significantly higher prevalence of BCV (72.5 and 65.5% respectively); almost the same two areas were identified as being high-prevalence clusters for BRSV (69.2 and 66.8% respectively). An area in south-east Sweden (Kronoberg-Blekinge) had lower prevalences for both infections than expected (23.8 and 20.7% for BCV and BRSV respectively). Another area in middle-west Sweden (Värmland-Dalarna) had also a lower prevalence for BRSV (7.9%). Areas with beef herd density > 10 per 100 km2 were found to be at significantly higher risk of being part of high-prevalence clusters.ConclusionThese results form a basis for further investigations of between-herds dynamics and risk factors for these infections in order to design effective control strategies.

Risk mapping of Rinderpest sero-prevalence in Central and Southern Somalia based on spatial and network risk factors

BackgroundIn contrast to most pastoral systems, the Somali livestock production system is oriented towards domestic trade and export with seasonal movement patterns of herds/flocks in search of water and pasture and towards export points. Data from a rinderpest survey and other data sources have been integrated to explore the topology of a contact network of cattle herds based on a spatial proximity criterion and other attributes related to cattle herd dynamics. The objective of the study is to integrate spatial mobility and other attributes with GIS and network approaches in order to develop a predictive spatial model of presence of rinderpest.ResultsA spatial logistic regression model was fitted using data for 562 point locations. It includes three statistically significant continuous-scale variables that increase the risk of rinderpest: home range radius, herd density and clustering coefficient of the node of the network whose link was established if the sum of the home ranges of every pair of nodes was equal or greater than the shortest distance between the points. The sensitivity of the model is 85.1% and the specificity 84.6%, correctly classifying 84.7% of the observations. The spatial autocorrelation not accounted for by the model is negligible and visual assessment of a semivariogram of the residuals indicated that there was no undue amount of spatial autocorrelation. The predictive model was applied to a set of 6176 point locations covering the study area. Areas at high risk of having serological evidence of rinderpest are located mainly in the coastal districts of Lower and Middle Juba, the coastal area of Lower Shabele and in the regions of Middle Shabele and Bay. There are also isolated spots of high risk along the border with Kenya and the southern area of the border with Ethiopia.ConclusionsThe identification of point locations and areas with high risk of presence of rinderpest and their spatial visualization as a risk map will be useful for informing the prioritization of disease surveillance and control activities for rinderpest in Somalia. The methodology applied here, involving spatial and network parameters, could also be applied to other diseases and/or species as part of a standardized approach for the design of risk-based surveillance activities in nomadic pastoral settings.

Population Characteristics of an Insular Fallow Deer (Dama dama L.) population on Little St. Simons Island, Georgia, U.S.A.

We describe population characteristics including body weight, antler measurements, pelage, population density, and reproduction of an introduced population of fallow deer ( Dama dama L. ) on Little Saint Simons Island, Georgia, USA. Between 2002 and 2006, we obtained data from 181 hunter-harvested deer. Adult males (n=45) averaged 48.8 kg ± 2.6 kg SE and females (n=34) averaged 35.7 kg ± 1.4 kg SE. Based on seasonal spotlight surveys sex ratios averaged 1.2:1 bucks to does, and recruitment estimated at 0.5 fawns per doe. Herd density was estimated at 47.3 deer/km 2 . Lek behavior was common and peak rut occurred in early to middle October. Fallow deer on LSSI demonstrated low body weights and antler measurements, high population density, and depressed reproduction. Based on these characteristics, further investigation is needed to assess herd health and food availability. A reduction in deer density may be warranted.

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection in wild boars

The Porcine Reproductive and Respiratory Syndrome (PRRS) is one of the economically most important swine diseases worldwide. The virus can be spread by viraemic and persistent infected pigs. Spread and infection are advanced in areas with high herd and population densities. As wild boars have been found seropositive for PRRSV in Germany, France, and the USA, exchange between wild and domestic pig populations may exist. However, comprehensive information on PRRSV infection in wild boars is presently not available. The aim of the current study was thus, to systematically study PRRSV infection in wild boars to provide information on spatiotemporal, host and viral effects. The study was based on 531 wild boars from 52 hunts in Germany (2004–2007). PRRSV infection was determined and strains (US/EU) were classified by PCR. A total of 15.9% of the wild boars were PRRSV-positive (US: 14.2%; EU: 6.2%), with remarkable effects of state (US: 5.1–46.2%; EU: 0–17.6%), season (0–36.5%) and tissue (lungs: 89%; tonsils: 11%). Prevalences did neither correlate with age or weight, nor with density of production units, domestic pigs or wild boars. Open reading frame (ORF) 1-sequences within EU- and US-strains did not differ among wild boar samples. Homologies between EU-samples/Lelystad-virus and US-samples/PRRSV-MLV virus were 99.3 and 97%, respectively. This is the first comprehensive evidence of PRRSV infection in wild boars. We conclude that there is only a weak relation between wild boar and domestic PRRSV infection.

Seroprevalence and risk factors for the presence of ruminant pestiviruses in the Dutch swine population

Swine can be infected with classical swine fever virus (CSFV), as well as ruminant pestiviruses: bovine viral diarrhoea virus (BVDV), and Border disease virus (BDV). Cross-reactions between pestiviruses occur, both regarding protective immunity and in diagnostic tests. The presence of BVDV and BDV in a swine population may thus affect the transmission of CSFV, but also the diagnosis of a CSFV infection. In this study, the seroprevalence against BVDV and BDV in two categories of swine, sows and finishing pigs, in the Netherlands was determined. Furthermore, several risk factors, associated with the presence of swine and ruminants on the same farm or in the immediate surroundings, were evaluated. In sows, the seroprevalence against BVDV was 2.5% on the animal level, and 11.0% on herd level. In finishing pigs these prevalences were 0.42% and 3.2%, respectively. Antibodies against BDV were found in three sows only. Risk factors, associated with a BVDV-seropositive status in breeding pigs, were the presence of cattle on the same premises and a high density of sheep and/or goats herds in a radius of 3 km. While BVDV and BDV hardly pose any threat to the swine population themselves, knowledge, and therefore regular monitoring, on the presence of these viruses in the swine population is important with respect to CSF eradication. It will allow for a better interpretation of diagnostic test results, both in terms of possible false positives and false negatives, but may also bring about additional measures or surveillance protocols in times of CSF outbreaks to avoid surprises caused by cross-reactivity with ruminant pestiviruses.

The range of influence between cattle herds is of importance for the local spread of Salmonella Dublin in Denmark

The objective of the study was to estimate the range of influence between cattle herds with positive Salmonella Dublin herd status. Herd status was a binary outcome of high/low antibody levels to Salmonella Dublin in bulk-tank milk and blood samples collected from all cattle herds in Denmark for surveillance purposes. Two methods were used. Initially, a spatial generalised linear mixed model was developed with an exponential correlation function to estimate the range of influence simultaneously with the effect of potential risk factors. An iteratively reweighted generalised least squares procedure was used as a second method for verifying the range of influence estimates. With this iterative procedure, deviance residuals were calculated based on a generalised linear model and the range of influence was estimated based on the residuals using an exponential semivariogram. The range of influence was estimated for six different regions in Denmark using both methods. The analyses were performed on data collected during 1 year after initiation of the Salmonella Dublin surveillance program providing herd classifications for the 4th year-quarter of 2003 and 2 years later for the 4th year-quarter of 2005. The prevalence of dairy herds with a positive Salmonella Dublin herd classification status in this period had decreased from 22.1 to 17.0%. In non-dairy herds, the prevalence was nearly unchanged during the same period (3.4 and 3.7% in 4th quarter of 2003 and 2005, respectively). For all cattle herds, the range of influence was 2.3–6.4 km in 2003 and 1.5–8.3 km in 2005. There seemed to be no association between the range of influence and the density of herds in the different regions.

Local spread of classical swine fever upon virus introduction into The Netherlands: Mapping of areas at high risk

BackgroundIn the recent past, the introduction of Classical Swine Fever Virus (CSFV) followed by between-herd spread has given rise to a number of large epidemics in The Netherlands and Belgium. Both these countries are pork-exporting countries. Particularly important in these epidemics has been the occurrence of substantial "neighborhood transmission" from herd to herd in the presence of base-line control measures prescribed by EU legislation. Here we propose a calculation procedure to map out "high-risk areas" for local between-herd spread of CSFV as a tool to support decision making on prevention and control of CSFV outbreaks. In this procedure the identification of such areas is based on an estimated inter-herd distance dependent probability of neighborhood transmission or "local transmission". Using this distance-dependent probability, we derive a threshold value for the local density of herds. In areas with local herd density above threshold, local transmission alone can already lead to epidemic spread, whereas in below-threshold areas this is not the case. The first type of area is termed 'high-risk' for spread of CSFV, while the latter type is termed 'low-risk'.ResultsAs we show for the case of The Netherlands, once the distance-dependent probability of local transmission has been estimated from CSFV outbreak data, it is possible to produce a map of the country in which areas of high-risk herds and of low-risk herds are identified. We made these maps even more informative by estimating border zones between the two types of areas. In these border zones the risk of local transmission of infection to a nearby high-risk area exceeds a certain level.ConclusionThe risk maps provide an easily understandable visualization of the spatial heterogeneities in transmission risk. They serve as a tool for area-specific designs of control strategies, and possibly also for spatial planning of areas where livestock farming is allowed. Similar risk maps can in principle be constructed for other highly-transmissible livestock infections that spread via neighborhood transmission.

Behavioural responses of feral goats (Capra hircus) to helicopters

Helicopters are commonly used for managing wildlife populations, but their effect on wildlife behaviour is poorly understood and often ignored by managers. Changes in behaviour can adversely affect wildlife, compromise assumptions of survey methods, and reduce the effectiveness of management operations. In this study, we investigated the behavioural responses of free-ranging feral goats to helicopters and the main determinants of alert behaviour in response to helicopters. Ground-based reporters made 784 observations of feral goat groups during 34 standardised helicopter surveys used to estimate abundance. Feral goats were often alert (44% of observations) and, in 31% of observations, moved (up to 1.5km) in response to helicopter over-flights but no feral goats were observed to be injured nor did any post-partum females desert their young in response to over-flights. Regression analyses indicated that the distance from the helicopter and prior activity were the most important factors influencing the extent of alert behaviour and the distance moved in response to helicopter disturbance. The responses of herds of goats in different home ranges were variable, and cumulative survey time, type of helicopter and the density of herds also influenced behavioural responses. Results indicated that, while long-term effects of helicopter disturbance on feral goat behavioural ecology are minimal, short-term changes in behaviour frequently occur and should be considered when using helicopters to manage feral goats.

Epidemiology of bovine brucellosis in Northern Ireland between 1990 and 2000

Between 1990 and 2000, 317 herds of cattle in Northern Ireland were identified as being seropositive to Brucella abortus, and 68 per cent of them were attributed to transmission from...

POPULATION DENSITY–PHYSICAL CONDITION RELATIONSHIPS IN WHITE-TAILED DEER

Abstract Management of white-tailed deer (Odocoileus virginianus) populations is important because of the popularity of this species for sport hunting and the ecological and economic damage deer can cause. Managers traditionally have collected data from hunter-harvested deer to provide information for making these harvest management decisions. We tested for predictive relationships between these data and density using long-term (15–31 years duration, median = 26 years) harvest data for 9 populations in the southeastern United States that spanned several physiographic provinces and a wide range of densities (3–32 deer/km2) that varied by a factor of 1.67–5.50 within populations over the study period. Mean annual harvest for these populations ranged from 265–5,242 animals (median = 626). We tested 8 physical and 3 recruitment measures commonly collected on harvested deer for their relationship to reconstructed herd density estimates using correlation and linear regression models. Yearling male dressed body ...

Epidemiological investigations of the 2001 foot-and-mouth disease outbreak in Argentina

A total of 2126 herds, an attack rate of 0.82 per cent, were affected during an epidemic of foot-and-mouth disease in Argentina in 2001. The spatial and temporal distribution of...

Spatial and temporal scaling of population density and animal movement: A power law approach

Many ecological patterns are sensitive to spatial and temporal scale, but no general protocol has emerged for dealing with scale-dependence. We suggest that power laws, regularly used in the study of body size, can be applied to many such features. We used these techniques to describe two scale-sensitive aspects of caribou (Rangifer tarandus): population densities of North American herds when measured at varying spatial extents, and speeds of radiotracked individuals when observed at varying temporal resolutions. Log-log regression was applied, based on the power formula, Y= cX z. Non-isometric relationships existed for abundance versus area of the population range, but the pattern differed between groups. For sedentary caribou, animal numbers increased more slowly than spatial extent (z = 0.503). For migratory populations, this relationship was hyper-allometric (z > 1), and densities were generally lower for continental herds compared to insular Newfoundland herds living without wolves (Canis lupus). We surmise that defining population bounds may be more subjective for sedentary herds; migratory herd densities may increase with increasing population size. Distance moved versus the interval between radiolocations was also non-isometric (z = 0.551). We standardized these movements to eliminate the confounding effects of scale, enabling comparisons across seasons and populations. The power formula helps to clarify the debate over the use of ratios, where variables may change non-isometrically. Power laws may enable the synthesis of ecological patterns sensitive to scales of space and time.

A decision-tree to optimise control measures during the early stage of a foot-and-mouth disease epidemic

A decision-tree was developed to support decision making on control measures during the first days after the declaration of an outbreak of foot-and-mouth disease (FMD). The objective of the tree was to minimise direct costs and export losses of FMD epidemics under several scenarios based on livestock and herd density in the outbreak region, the possibility of airborne spread, and the time between first infection and first detection. The starting point of the tree was an epidemiological model based on a deterministic susceptible–infectious–recovered approach. The effect of four control strategies on FMD dynamics was modelled. In addition to the standard control strategy of stamping out and culling of high-risk contact herds, strategies involving ring culling within 1 km of an infected herd, ring-vaccination within 1 km of an infected herd, and ring-vaccination within 3 km of an infected herd were assessed. An economic model converted outbreak and control effects of farming and processing operations into estimates of direct costs and export losses. Ring-vaccination is the economically optimal control strategy for densely populated livestock areas whereas ring culling is the economically optimal control strategy for sparsely populated livestock areas.

Seroprevalence of Actinobacillus pleuropneumoniae serovars 2, 3 and 9 in slaughter pigs from Belgian fattening farms.

Fifty randomly selected fattening pig herds were studied to investigate the epidemiological characteristics of infections with Actinobacillus pleuropneumoniae serovars 2, 3 and 9, and to identify risk factors for their within-herd seroprevalences. Information about 13 farm characteristics was obtained by means of a questionnaire and used to assess potential risk factors for the percentage of slaughter pigs with antibodies against each of the three serovars. The presence of antibodies was measured with an indirect ELISA. The median within-herd seroprevalence for serovar 2 was 58 per cent (range 0 to 100 per cent), for serovar 3, 53 per cent (range 10 to 95 per cent), and for serovar 9, 35 per cent (range 5 to 100 per cent). All but one farm tested positive for A pleuropneumoniae serovar 2, and all the farms were positive for A pleuropneumoniae serovars 3 and 9. There was a positive association (P < 0.05) between each pair of serovars. The within-herd seroprevalence of serovar 2 was significantly associated with the density of pig herds in the municipality (odds ratio [OR] = 1.60; P < 0.05) and with the absence of preventive medication at the start of the fattening period (OR = 2.77; P < 0.10). No significant risk factors were found for serovar 3. The percentage of pigs positive for serovar 9 was significantly associated with a slaughter date in June (OR = 2.30; P < 0.10) and with herds in which the finishing houses were not divided into separate compartments (OR = 2.99; P < 0.05).

Empirical and theoretical evidence for herd size as a risk factor for swine diseases.

Herd size is frequently studied as a risk factor for swine diseases, yet the biological rationale for a reported association with herd size (whether positive or negative) is rarely adequately discussed in published epidemiological studies. Biologically plausible reasons for a positive association between herd size and disease include a greater risk of introduction of pathogens from outside the herd, greater risk of transmission of pathogens within and among herds when the herd is large, and effects of management and environmental factors that are related to herd size. However, compared with owners of small herds, owners of large herds might more frequently adopt management and housing practices that mitigate this theoretically increased risk. We used studies of pleuritis, pneumonia and pseudorabies to describe the epidemiological issues involved in evaluations of the relationship between management factors, herd size and disease. In future studies, we recommend that (i) herd size be measured in a way that best characterizes the true population at risk; (ii) studies that evaluate management-related risk factors should account for herd size wherever possible; (iii) population-based studies of the interrelationships among management factors and between management factors, herd size, herd density and pig density be done; (iv) likely biological reasons for any herd-size effect be postulated; and (v) the distribution of herd sizes in the source population and the study sample be described.

Risk factors for infection of sow herds with porcine reproductive and respiratory syndrome (PRRS) virus

In 1992, the porcine reproductive and respiratory syndrome virus (PRRSV) of European type (PRRSV-EU) was introduced in Denmark. By 1996, the virus had spread to approximately 25% of the Danish herds. In January 1996, a modified-live vaccine based on the American type of the virus (PRRSV-US) was used in replacement boars for Danish artificial insemination (AI) centres and from July 1996, the vaccine was used in PRRSV-EU infected herds for prevention of disease. Soon after vaccine introduction, PRRSV non-infected herds experienced outbreaks of disease due to infection with PRRSV-US. In this study, we investigated the risk factors (biosecurity level, animals, exposure from PRRSV-US-infected neighbour herds, semen, herd size, pig density and herd density) for infection with PRRSV-US in a cohort of 1071 sow herds; we used a nested case–control study. The retrospective observation period lasted from June 1996 (when they all were non-infected) to October 1997. Seventy-three non-vaccinated, closed sow herds became infected with the vaccine strain during this period. Each case herd was matched with two control herds from the cohort (controls had not been infected at the time of infection in the case herds). The data were analysed using a Cox-regression model. The hazard of infection increased significantly with exposure from PRRSV-US-infected neighbouring herds, purchase of animals from herds incubating PRRSV-US infection, increasing herd size and purchase of semen from boars at PRRSV-US-infected AI centres. The results are consistent with the modified-live vaccine strain spread to other herds by trade with animals and semen and by neighbour (area) transmission. We suggest that virus spread by aerosols was a frequent mode of transmission.

REGULAR ARTICLES / ARTICLES RÉGULIERSFactors affecting the distribution and transmission ofElaphostrongylus rangiferi(Protostrongylidae) in caribou (Rangifer tarandus caribou) of Newfoundland, Canada

Elaphostrongylus rangiferi was introduced to caribou (Rangifer tarandus caribou) of Newfoundland by infected reindeer (R. t. tarandus) from Norway and has caused at least two epizootics of cerebrospinal elaphostrongylosis (CSE), a debilitating neurologic disease. In an attempt to understand the conditions necessary for such outbreaks, we examined the effects of herd density and climatic factors on parasite abundance. The abundance of E. rangiferi was represented by counts of first-stage larvae in feces collected from young caribou (calves and yearlings) in 7 distinct caribou herds in Newfoundland. Abundance of E. rangiferi was highest in February and in the Avalon (632 ± 14 (mean ± SE)) and St. Anthony (526 ± 145) herds, the 2 herds in which CSE was most frequently reported. Mean abundance in February samples from young animals correlated positively with mean annual minimum temperature (rS= 0.829, df = 6, P = 0.04) and the number of days per year above 0°C (rS= 0.812, df = 6, P = 0.05) and negatively with mean summer temperatures (rS= –0.830, df = 6, P = 0.04). Results suggest that abundance of E. rangiferi and the likelihood of cases of CSE are increased by moderate summer temperatures suitable for the activity and infection of gastropod intermediate hosts and by mild winters with little snow that extend the transmission period. Abundance of larvae was not correlated with herd density. Animals in all 7 herds also had the muscle worm Parelaphostrongylus andersoni, a related nematode with similar dorsal-spined larvae. In 2 additional herds (Cape Shore and Bay de Verde), P. andersoni occurred alone and larvae were passed only by young caribou. In herds with dual infections, numbers of P. andersoni larvae were depressed, declined more quickly in young animals, and were considered to be present in only low numbers in February samples used for E. rangiferi analysis. Upon initial infection, young caribou develop a resistance to E. rangiferi that prevents or reduces reinfection later in life. This was demonstrated by examining the brains of caribou for recently acquired worms, which must develop there for up to 90 days before continuing their tissue migration into the skeletal muscles. Recent infections were detected in only calves and yearlings in all herds with E. rangiferi except the Avalon herd, where developing worms were also found on the brains of older caribou. The infection of older animals in the Avalon herd may reflect a lower immunocompetence of a naive herd that has only recently been exposed to E. rangiferi.

An extended state-transition model for foot-and-mouth disease epidemics in France

The dynamics of foot-and-mouth disease epidemics in France was examined through simulations based on an extended state-transition model. Contagion modelling depended on specific parameters: the so-called dissemination rates. Estimation of these parameters relied on a specific discrete-event simulation model. We were able to address the problem of the hidden spread of the epidemic, before the first outbreak is diagnosed. Furthermore, we took into account the silent development of the disease in affected herds before the diagnosis. The effect of control measures such as the active search for secondary outbreaks could thus be studied. We used the model to compare the development of FMD epidemics in two very different French regions and for different control strategies implemented by the animal-health authorities. These strategies gave similar results in a low herd density area, whereas in a high herd density area, the slaughter of contact herds greatly improved the stamping-out strategy. Finally, key parameters of the model were detected through a sensitivity analysis.

Herd factors associated with the seroprevalences of four major respiratory pathogens in slaughter pigs from farrow-to-finish pig herds.

The objective of this study was to investigate sero-epidemiological aspects of Mycoplasma hyopneumoniae (Mh), influenza H1N1 and H3N2 viruses and Aujeszky disease virus (ADV) in fattening pigs from 150 randomly selected farrow-to-finish pig herds. Different herd factors were examined as potential risk indicators for the percentage of pigs with antibodies against the 4 pathogens. The median within-herd seroprevalences of the pathogens were: Mh 76%, H1N1 100%, H3N2 40% and ADV 53%. There was a positive association between the seroprevalences of both influenza viruses, and a negative association between the seroprevalences of ADV and H1N1. The percentage of pigs seropositive for Mh increased with the purchase of gilts and with the season (slaughter date in March-April). The within-herd seroprevalences of both influenza viruses were higher in the case of a higher density of pig herds in the municipality. A higher number of fattening pigs per pen additionally increased the risk of being seropositive for H3N2. The percentage of pigs with anti-gE-antibodies against the wild type ADV increased with higher airspace stocking density in the finishing unit, increasing herd size, increasing number of pig herds in the municipality and slaughter date in March-April. Increased seroprevalences for these 4 respiratory pathogens were mostly associated with pig density in the herd and its vicinity, the winter period, and with the purchase of gilts. Purchase of gilts, number of fattening pigs per pen and airspace stocking density are risk factors that can be managed directly by farmers striving to attain a high respiratory health status of pigs.